Rooftop Greenery – the horizontal dimension
Rooftop greenery refers to the greening efforts and landscaping on rooftop surfaces.
In some instances, rooftop greenery can maintain and even enhance the biodiversity of a site through the conscious selection of indigenous vegetation species and landscape materials, and a greening strategy that respects the existing ecology.
Rooftop greenery, when appropriately designed, can improve building performance – enhancing the durability of water-proof barriers, reducing rainwater run-off, providing better heat/sound insulation, mitigating the heat island effect by lowering urban air temperature, etc.
Additional features, such as rainwater-harvesting system, etc, can potentially be incorporated into the rooftop greenery design strategy.
In general, there are two main categories of rooftop greenery – green roof (extensive rooftop greenery) and roof garden (intensive rooftop greenery).
Green Roof (extensive rooftop greenery)
A green roof is one that has been extensively covered with vegetation. Very often, the vegetation support layer is installed over an integrated irrigation and drainage system, with root barrier and waterproofing membrane protecting the roof surface.
The green roof is a suitable rooftop greenery strategy for greening the rooftop of existing buildings / structures. With its thinner vegetation support layer, it is lightweight compared to the heavier roof garden, but supports a limited range of vegetation species – with a preference for drought-tolerant vegetation species with shallow roots.
A green roof is a potential value-adding feature to an existing or new building. It generally requires minimal maintenance. Access is usually strictly for periodic maintenance.
Components of the Green Roof
There are five basic components, namely plants layer, substrates and media layer, filter Layer, drainage and storage cells layer, and protection membrane and root barrier layer. Though there are different systems and technology used in the industry, all of them are based on the same component principles, but designed differently in order to meet the various site conditions and needs.
Typology
Generally, most systems fall into 2 categories – in situ green roof and modular green roof.
An in situ green roof is customised to the roof site area and involves assembling the green roof components in sheet layers, directly across the roof. Being a closer variation of the traditional intensive rooftop garden, it is commonly used and well established.
The modular green roof is a more recent product of green roof system development. It involves the modularisation of green roof components into standard small trays (sizes typically range from 0.25 to 2 square meters). Each tray is equipped with a filter layer, drainage and storage cells to hold the substrate and media for the plants.
Type of plantings
The criteria for green roof plant selection should be based on the type of systems, intended planting concept, environmental factors, budget and expected level of maintenance.
Plants should typically be able to tolerate drier conditions and thrive in high daytime temperature, intense sunlight, and low soil moisture. Plants that have high water use efficiency are those that employ the Crassulacean Acid Metabolism mode of photosynthesis, or CAM for short. These plants will close the stomata during daytime, when evaporative loss of water from the leaves tends to be greatest, and keep the stomata open at night to absorb carbon dioxide for storage and use in photosynthesis. CAM plants also possess morphological adaptations such as thick cuticles, succulence, low surface-to-volume ratios and reduced stomata size and/or frequency that help to reduce water loss.
CAM plants are very widespread and they are represented in 33 families encompassing 18,900 plant species. Some commonly used species include Sedum, Neoregelia and Portulaca cultivars.
Roof Garden (intensive rooftop greenery)
A roof garden is usually designed as an accessible outdoor space for leisure and has a more elaborate and aesthetic layout than a green roof. The vegetation support medium can be of varying depths to support a diverse range of vegetation species. Very often, this is installed with an integrated irrigation and drainage system, with root barrier and waterproofing membrane protecting the roof surface.
A roof garden, with potentially deeper soil depths to support a wide range of vegetation species, is generally heavier than a green roof. It is therefore more suitable for new buildings, where the structures of the building can be appropriately designed to cater for the higher overall loading.
It should be noted that potted plants are generally not considered as permanent vegetation on the roof. Thus, will not be defined as an integral part of a roof garden.
With a potentially a wider range of landscaping materials and vegetation species, a roof garden generally requires more maintenance.
Components of the Roof Garden
Similar to the green roof, the five basic components of a roof garden system are namely the plants layer, substrates and media layer, filter layer, drainage cells layer, and protection membrane and root barrier layer. The main difference is the more substantial thickness of the substrate and media layer, which ranges from 200mm for ground cover and up to 1m or more for small trees.
Types of plantings
The criteria for roof garden plant selection should be based on the intended planting concept, environmental factors, budget, and expected level of maintenance.
Plant selection for roof garden is much broader than that of green roof, ranging from ground cover to small trees. The placement of the bulkier landscape components such as trees should consider the efficient transfer of the load through the building structures.